Low-grade follicular non-Hodgkin's lymphoma (NHL) is characterized by an indolent clinical course where >85% of patients fail to achieve long-term remissions with any therapeutic modality thus far developed. We have developed an immunotoxin (IT) consisting of the RFB4 anti-CD22 monoclonal antibody (MAb) linked to deglycosylated ricin-A-chain (dgRTA). This IT has been tested in SCID/lymphoma mice, in primates, and has been administered to >75 patients with multiply relapsed NHL in Phase I clinical trials. The response rate has been 40% and the dose limiting toxicity (DLT) has been vascular leak syndrome (VLS). VLS has also been the DLT in -250 patients whom we have treated with IT-dgAs against CD19, 25 and 30. Because of VLS, our enthusiasm for evaluating this and other ITs in Phase II trials has been significantly dampened. To address the problem of VLS, we developed in vitro and in vivo models of VLS and identified the sequence in RTA responsible for this side effect. We then generated a panel of 16 RTA mutants, expressed them in E. coli, purified and evaluated them. One mutant made an IT which was as active as an IT prepared with dgRTA but did not induce VLS in our animal models. We now seek funding to carry out a Phase I trial with this third generation IT to determine if we have "solved" the VLS problem in humans. The first year will be spent preparing the IT, carrying out all the FDA-required testing, and filing the IND. The Phase I trial will be carried out in the second and third years by Dr. E. Sausville at the NCI. It will be a dose escalation study in 40 patients. Half of the patients will have circulating tumor cells (CTCs) and half will not. Our objectives will be to determine 1) the maximum tolerated dose (MTD) of this third generation IT in patients with relapsed low or intermediate grade NHL who do, or do not have CTCs, 2) the pharmacokinetics (PK) immunogenicity, clinical response, and any changes in levels of circulating cytokines. 3) whether there are correlations between PKs, changes in cytokines, toxicity, and response. Our data will be compared with those obtained in the past clinical trials with the second generation IT, RFB4-dgRTA.